This invention relates to Fe--Ni alloys with improved press punchability which are suitable as materials for electron gun parts, such as electrodes for electron guns. The invention also relates to punched electron gun parts, typically electron gun electrodes, obtained by blanking a stock of such alloy and punching very small holes in the blanks for passage of electron beams therethrough.
FIG. 1 shows a cross section of a color picture tube of the shadow mask type already known in the art. A panel 1 is coated on the back side with a phosphor film 2 that generates the three primary colors of red, green, and blue. In the neck are housed electron guns 4 that emit electron beams 3. The electron beams 3 are deflected in scanning by a deflection yoke 5. The numerals 6 and 7 indicate a shadow mask and a magnetic shield, respectively.
FIGS. 2A and 2B are perspective and cross sectional (along line I--I' in FIG. 2(a)) views, respectively, of an electrode (grid electrode) 10 as an example of punched part to be fitted in the electron guns 4. The electrode 10 acts to control the thermionic emission of electrons from the cathodes in the electron guns, form electron beams, and modulate the flow rate of electrons. The electrode 10 has very small holes 10a, 10b, and 10c made by coining and punching so as to allow red, green, and blue color-generating beams, respectively, to pass through them.
In general, the electron gun parts for use in picture tubes and the like are completed by blanking and punching, with or without coining, a sheet of nonmagnetic stainless steel about 0.05 to 0.5 mm thick.
Nonmagnetic stainless steels are well-known materials in common use for electron gun parts of picture tubes and the like. In the manufacture of the electrode that accelerates the electrons emitted from the cathodes of electron guns, however, more weight has recently been put on lower thermal expansion coefficient than on nonmagnetic properties. With the advent of higher refinement, higher performance picture tubes for computer displays and the like in recent years, it has been noted that delicate dimensional changes with thermal expansion of electrode parts influence the quality of pictures (color purity) on the panel 1 (see FIG. 1). To cope with the situation, Fe--Ni alloys having low thermal expansion properties, notably Fe-42% Ni alloy (42 Alloy), have come into use as electrode materials. The 42 Alloy of the prior art, however, presents a burr problem. As blanks of the alloy for electrodes are punched with a pattern of very small holes 10a, 10b, and 10c each, the punching dies produce burrs B on the edges 10e of the holes where they have forced slugs down (see FIGS. 2A and 2B). The burrs that result from the punching not only have adverse effects upon the control of electron beams but also cause the emission of unwanted electron, both of which could be fatal defects for electron guns. The tendency toward a higher level of refinement of picture tubes is making the requirement for the reduction of burrs generated in electron gun parts more and more exacting.
Proposals for improving the punchability of Fe--Ni alloys have hitherto been made, for example, in Japanese Patent Application Kokai Nos. 6-184703, 6-122945, 7-3400, and 7-34199.
Of those proposals, Kokai No. 6-184703 specifies an S content in the range of 0.002 to 0.05% and disperses S or an S compound along grain boundaries or within grains in the alloy stock. However, the addition of S, a mere free-cutting element, in a specified percentage cannot be deemed adequate for the control of burrs to most precise specifications in the recent and future press working parts.
Kokai Nos. 6-122945, 7-3400, and 7-34199 propose adding such strengthening elements as Ti, Nb, V, Ta, W, or/and Zr to the alloy for sufficient hardness and proper embrittlement to suppress burring. The enhanced hardness, in turn, poses a problem of shortened punching die life.
This invention has for its object to settle the afore-described problems of the prior art and provide Fe--Ni alloys for electron gun parts improved in punchability without attendant shortening of die life, and also provide punched parts for electron guns, typically electron gun electrodes, made of the alloys by punching and having reduced burrs.